Cardiovascular Physiologic Parmaters

Canine and Feline Cardiovascular Physiology for Veterinary Anesthesiology #

Introduction #

Understanding the normal cardiovascular physiology of dogs and cats is essential for veterinary anesthesiologists to anticipate and manage cardiovascular changes during anesthesia. This article summarizes key cardiovascular parameters, homeostatic mechanisms, and species-specific considerations relevant to veterinary anesthesia practice.

Normal Cardiovascular Parameters #

Heart Rate #

• Dogs:
  • Small breeds: 90-120 beats per minute (bpm)
  • Medium breeds: 70-110 bpm
  • Large breeds: 60-90 bpm
  • Giant breeds: 50-80 bpm
  • Athletic/working dogs: 40-60 bpm at rest
  • Puppies: 120-220 bpm
• Cats:
  • Adults: 140-220 bpm
  • Kittens: 180-240 bpm
  • Sinus arrhythmia less pronounced than in dogs

Blood Pressure #

• Dogs:
  • Systolic: 100-160 mmHg
  • Diastolic: 60-100 mmHg
  • Mean arterial pressure (MAP): 80-120 mmHg
  • Central venous pressure (CVP): 0-5 cmH₂O
• Cats:
  • Systolic: 110-170 mmHg
  • Diastolic: 70-110 mmHg
  • Mean arterial pressure (MAP): 90-130 mmHg
  • Central venous pressure (CVP): 0-5 cmH₂O

Cardiac Output #

• Dogs:
  • 100-200 mL/kg/min
  • Approximately 2-4 L/min for a 20 kg dog
  • Cardiac index: 3.5-5.5 L/min/m²
• Cats:
  • 150-300 mL/kg/min
  • Approximately 300-600 mL/min for a 4 kg cat
  • Cardiac index: 3.0-5.0 L/min/m²

Stroke Volume #

• Dogs: 1.3-2.6 mL/kg (25-50 mL for a 20 kg dog)
• Cats: 1.0-2.0 mL/kg (4-8 mL for a 4 kg cat)

Systemic Vascular Resistance (SVR) #

• Dogs: 1800-2500 dyn·s/cm⁵
• Cats: 2000-2800 dyn·s/cm⁵

Pulmonary Vascular Resistance (PVR) #

• Dogs: 200-300 dyn·s/cm⁵
• Cats: 200-400 dyn·s/cm⁵

Ejection Fraction #

• Dogs: 50-65%
• Cats: 55-70%

Fractional Shortening #

• Dogs: 25-45%
• Cats: 30-55%

Electrocardiographic (ECG) Parameters #

P Wave #

• Dogs:
  • Duration: 0.04-0.05 sec
  • Amplitude: ≤ 0.4 mV (Lead II)
• Cats:
  • Duration: 0.03-0.04 sec
  • Amplitude: ≤ 0.2 mV (Lead II)

PR Interval #

• Dogs: 0.06-0.13 sec
• Cats: 0.05-0.09 sec

QRS Complex #

• Dogs:
  • Duration: 0.04-0.06 sec
  • Amplitude: ≤ 3.0 mV (Lead II)
• Cats:
  • Duration: 0.03-0.04 sec
  • Amplitude: ≤ 0.9 mV (Lead II)

QT Interval #

• Dogs: 0.15-0.25 sec (varies with heart rate)
• Cats: 0.12-0.18 sec (varies with heart rate)

Hemodynamic Regulation #

Autonomic Nervous System Influence #

summary

Baroreceptor Reflex #

• Location:
  • Carotid sinus
  • Aortic arch
  • Cardiopulmonary receptors
• Response to Decreased Arterial Pressure:
  • Increases sympathetic output
  • Decreases parasympathetic output
  • Dogs demonstrate more pronounced baroreceptor reflex than cats
• Response to Increased Arterial Pressure:
  • Decreases sympathetic output
  • Increases parasympathetic output

Chemoreceptor Reflex #

• Location:
  • Carotid bodies
  • Aortic bodies
• Response to Hypoxemia:
  • Increases ventilation
  • Causes peripheral vasoconstriction
  • Increases heart rate (mild effect)
  • PaO₂ threshold for activation: approximately 60 mmHg

Renin-Angiotensin-Aldosterone System (RAAS) #

• Activation:
  • Decreased renal perfusion pressure
  • Sympathetic stimulation
  • Decreased sodium delivery to macula densa
• Effects:
  • Vasoconstriction via angiotensin II
  • Increased aldosterone secretion
  • Sodium and water retention
  • More rapidly activated in cats than dogs

Cardiovascular Response to Anesthetic Drugs #

Inhalant Anesthetics #

• Common Effects:
  • Dose-dependent myocardial depression
  • Vasodilation
  • Baroreceptor reflex depression
  • Sensitization of myocardium to catecholamines
• Species Differences:
  • Cats: More sensitive to myocardial depression
  • Sighthounds: Prolonged recovery due to high muscle:fat ratio
  • Brachycephalic breeds: Greater risk of bradycardia and hypotension

Injectable Anesthetics #

• Propofol:
  • Myocardial depression
  • Peripheral vasodilation
  • Minimal baroreceptor depression
  • Faster redistribution in dogs compared to cats
• Alfaxalone:
  • Minimal cardiovascular depression at clinical doses
  • Preserves baroreceptor function
  • Similar pharmacokinetics between species
• Ketamine:
  • Increases heart rate and blood pressure
  • Maintains cardiac output
  • Direct myocardial depression masked by sympathetic stimulation
  • Less pronounced sympathetic effects in cats with depleted catecholamine stores
• Opioids:
  • Bradycardia (μ-agonists)
  • Minimal direct myocardial effects
  • More pronounced vagal effects in dogs than cats
  • Morphine: histamine release more significant in dogs
• Alpha-2 Agonists:
  • Biphasic blood pressure response
  • Initial hypertension (peripheral vasoconstriction)
  • Prolonged hypotension (central sympatholytic effect)
  • Profound bradycardia
  • Decreased cardiac output (40-50%)
  • More pronounced effects in cats compared to dogs

Special Physiologic Considerations #

Breed-Specific Variations #

• Brachycephalic Breeds:
  • Higher vagal tone
  • Predisposition to bradyarrhythmias under anesthesia
  • Increased risk of hypotension due to hypoxemia and respiratory acidosis
• Sighthounds:
  • Lower body fat percentage
  • Altered drug metabolism and distribution
  • Higher hematocrit (40-65%)
  • Increased oxygen-carrying capacity
  • Baseline lower heart rates
• Giant Breeds:
  • Predisposition to dilated cardiomyopathy
  • Increased incidence of atrial fibrillation
  • Physiologic cardiac murmurs common

Pediatric Patients #

• Neonatal Dogs and Cats (0-2 weeks):
  • Immature baroreceptor reflexes
  • Limited cardiac contractile reserve
  • Fixed stroke volume (cardiac output dependent on heart rate)
  • Increased sensitivity to volume changes
• Juvenile Dogs and Cats (2-12 weeks):
  • Higher cardiac output to body weight ratio
  • Higher metabolic demands
  • Developing autonomic control
  • Greater heart rate response to stress

Geriatric Patients #

• Cardiovascular Changes:
  • Decreased baroreceptor sensitivity
  • Reduced β-adrenergic responsiveness
  • Increased vascular stiffness
  • Decreased maximum heart rate
  • More pronounced in dogs than cats

Pregnancy #

• Hemodynamic Adaptations:
  • Increased cardiac output (30-50%)
  • Expanded blood volume (40% by term)
  • Decreased systemic vascular resistance
  • Increased heart rate (10-15%)
  • Aortocaval compression risk in late gestation

Monitoring Implications #

Minimal Anesthetic Monitoring #

• Heart Rate and Rhythm:
  • Bradycardia: HR < 60 bpm in dogs, < 100 bpm in cats
  • Tachycardia: HR > 160 bpm in dogs, > 240 bpm in cats
• Blood Pressure:
  • Hypotension: MAP < 60 mmHg
  • Severe hypotension: MAP < 50 mmHg
  • Hypertension: MAP > 140 mmHg
• Tissue Perfusion:
  • Mucous membrane color
  • Capillary refill time (normal: 1-2 seconds)
  • Peripheral pulse quality
  • Temperature gradient (core to peripheral)

Advanced Monitoring Considerations #

• Echocardiography:
  • Dynamic assessment of preload, contractility, and valvular function
  • Particularly valuable in patients with cardiac disease
• Central Venous Pressure:
  • Normal range: 0-5 cmH₂O
  • Interpretation affected by thoracic pressure and right heart function
• Cardiac Output Measurement:
  • Thermodilution
  • Lithium dilution
  • Pulse contour analysis
  • Valuable for high-risk cases and shock states

Conclusion #

A thorough understanding of canine and feline cardiovascular physiology is fundamental for veterinary anesthesiologists. Species and breed variations, as well as individual patient factors, significantly influence cardiovascular responses to anesthesia. Careful pre-anesthetic assessment, appropriate drug selection, and vigilant monitoring are essential for maintaining cardiovascular stability during anesthesia in dogs and cats.

References #

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